1. Field of the Invention
This invention relates generally to the liquid crystal displays and more particularly to liquid crystal displays for which a wide viewing angle is required.
2. Description of the Related Art
In the related art, the use of liquid crystal displays have gained increasing acceptance in a wide variety of applications as properties of the liquid crystal displays, such as resolution response, etc., have improved. The liquid crystal displays have been particularly useful in avionics wherein the amount of space available is limited and, by replacing the relatively bulky cathode ray tube display with a liquid crystal display, additional processing apparatus can be incorporated in the display.
The liquid crystal displays have, in the past, been limited by a relatively narrow field of view. This limited field of view has compromised the usefulness of liquid crystal displays particularly in the field of avionics.
The problem has been addressed in U.S. patent application Ser. No. 07/452,672 by providing that the thickness of liquid crystal material be at a minimum of optical transmission. This minimum will be a function of the wavelength of light transmitted therethrough. The thickness of the liquid crystal material through which the radiation passes is then made smaller than the thickness at which the transmission minimum is found. In this manner, the minimum for the radiation is found at an angle with respect to an axis through the liquid crystal, i.e., because of the increased path. While unwanted radiation transmission can occur on axis, the net result is that the contrast ratio provides a suitable viewing over a wider angle of view. For a color display, each optical component will have a different optimal thickness of liquid crystal material. This type of display is typically described as a "multi-gap" liquid crystal display.
However, another problem was not addressed by the thickness of the liquid crystal cell is the angular dependence of the off-axis transmitted radiation. At an off-axis angle, as a display is rotated, an image can become blurred and difficult to view for certain angles. This image degradation is a result of the change in the birefringence parameter of the liquid crystal material and the optical path length through the liquid crystal material. The image degradation is important for displays in which the liquid crystal cells are relatively thick. U.S. Pat. No. 4,385,806 by Fergason describes a technique involving the use of retardation plates for use in liquid crystal displays in which the effects angular dependence of the display can be reduced. However, the display described by the Fergason reference is a monochromatic display and the problems related to color displays are not addressed.
A need has been felt for a display in which both the angle of view and the angular dependence of a polychromatic liquid crystal display can be achieved.